Set the presentation width to 80 characters

That's the ideal line length in a git commit. I can make the
presentation match it.

Author: danott

style.css

@@ -157,7 +157,7 @@ html {
 
 body {
   font-size: 1rem;
-  max-width: 50rem;
+  max-width: 80ch;
   margin: 2rem auto;
 }